public class T3_3_2{
    public static void main(String s[]){
	double sum=0,j=1;
	int i =0;
	do{
	   i=i+1;
	   j=j*i;
	   sum=sum+1.0/j;
	}while(i<20);
	System.out.println(sum);
    }
}
	  